APCO Project 25 (P25) radio networks are perhaps the most widely-deployed digital radio technology currently in use by emergency first-responders across the world. This paper presents the results of an investigation into the security aspects of the P25 communication protocol. The investigation uses a new software-defined radio approach to expose the vulnerabilities of the lowest layers of the protocol stack. We identify a number of serious security flaws which lead to practical attacks that can compromise the confidentiality, integrity and availability of P25 networks.